Reef-building corals are animals (relatives of jellyfish and anemones) that have strong calcium carbonate skeletons. They form a symbiotic relationship with zooxanthellae algae that live in the coral’s tissues, and also give them their brightly coloured pigmentation. The photosynthetic algae help the coral to remove wastes and produce essential nutrients for the coral to live and grow, while the coral provides shelter for the algae and compounds they need for photosynthesis. This relationship ensures coral can be sustained in nutrient poor areas.
Therefore, a bleached coral is not necessarily dead. If you see a bleached coral, you are watching a coral that has lost its feeding companion, due to a broken symbiosis. Without algae, the coral has to rely on other (heterotrophic) methods of feeding.
Changes in conditions of the ocean, such as a change in nutrients, light or temperature, causes zooxanthellae algae to become stressed and release toxins. In response, the coral host may expel the zooxanthellae algae or the algae, which find their environment no longer suitable, may expel themselves, leaving the coral to appear totally white or ‘bleached’. If the change in condition is brief enough, coral can recover their algae. However, if the change is long lasting, the corals become weakened, more susceptible to disease, and will eventually die without zooxanthellae to support their metabolic processes.
Coral bleaching events have been increasing in both frequency and severity worldwide over the past 20 years. In the Caribbean, scientists have watched as coral coverage has deteriorated dramatically. A 2013 report from the International Union for Conservation of Nature (IUCN), estimated that total live coral cover declined from 58% in 1973 to 8% in 2012. Coral bleaching is one of the main contributing factors to a weakened reef system, adding to the other negative impacts that humans have on the reef. Two species of Acropora coral (Elkhorn and Staghorn) in the region have been listed as endangered, a classification reserved for species in danger of extinction.
When the team arrived in Bermuda in September 2013 NOAA's Coral Reef Watch had published a "Level 1 Bleaching Alert" for the area, this meant the thermal state of the region would likely experience bleaching. The team did indeed notice significant bleaching of 'fire corals'. "It was a surreal sight - coral that should have been brown and surrounded by fish was pure white and standing alone - it was stunningly beautiful yet very sad to witness. It's an ominous sign for what is coming in the future due to the delayed impacts of greenhouse gas emissions" - Richard Vevers, Project Director of the Catlin Seaview Survey.
In deeper water the impacts of the Level 1 Bleaching event are far less obvious but there was still widespread bleaching across several species of coral.
Thankfully, since the team have left the region the Bleaching Alert has been down-graded which should allow the affected corals to recover.